In recent times more and more emphasis is being placed on the flame retardancy of construction materials, such as employed both in interior and in exterior applications. Cellulosic hardboards and particle boards are widely utilized in the construction of buildings and due to their high organic content they present a potential fire hazard. This undesirable property has been long recognized and numerous attempts have been made to impart flame retardance to these boards, with varying degrees of success.
Early attempts described in U.S. Pat. Nos. 1,907,711 and 2,108,761 involve the use of inorganic fillers, such as alumina hydrate, asbestos, Fuller's earth and vermiculite as flame retarding agents; more recent patents, such as U.S. Pat. No. 3,873,662 and Canadian Pat. No. 971,049 recommend the use of sodium borate and mono and dibasic ammonium phosphates for the same purpose. It has also been suggested that condensation products of the reaction of formaldehyde with a nitrogen-containing compound such as urea, melamine, dicyandiamide or quanidine will impart flame retardancy to cellulosic boards when incorporated in such boards with small quantities of an inorganic filler, for example up to 25% by weight alumina hydrate, calculated on the weight of the finished board.
Thus, U.S. Pat. Nos. 2,611,694 and 2,680,102 recommend the use of flame-retarding compositions consisting of a urea or melamine-formaldehyde resin in combination with a chlorinated paraffin and small quantities of alumina hydrate. U.S. Pat. Nos. 2,488,034, 2,628,946, 2,632,743, 2,872,355, 2,917,408, 3,159,503, 3,372,131, 3,479,211, 3,786,041, 3,832,316, 3,887,511 and Canadian Pat. No. 972,227 employ as flame-retarding agents the above-mentioned melamine-formaldehyde or urea-formaldehyde condensation products in combination with phosphoric acid or the phosphate salt of the nitrogen-containing constituent of the condensate. Further attempts in rendering cellulosic products, such as hardboards and particle boards, flame retardant are described in Japanese Published Specification No. 92,139 of 1974, which was published on Sept. 2, 1974. In this reference a cellulosic board is coated on its surface with a composition containing a urea or melamine-formaldehyde precondensate, ammonium phosphate or an organic phosphate salt, a polyhydric alcohol and a metal hydroxide filler, such as alumina hydrate, zinc hydroxide or ferric hydroxide. The coating is applied on the board surface and then is covered with a water-proofing material to preserve the effectiveness of the flame-retarding layer. Although the art in flame-retarding cellulosic boards is extensive, the available references fail to suggest the incorporation of the present flame retarding composition in cellulosic boards which consists of the synergistic combination of alumina hydrate with melamine-formaldehyde-phosphate or with urea-formaldehyde-phosphate; or with dicyandiamide-formaldehyde-phosphate. The extent of the published prior art in the field of rendering cellulosic boards flame retardant not only indicates the importance of the problem i.e., the need for improved flame retardancy, but it also shows that the compositions described do not meet in all respects the stringent flame retardance standards presently required from products to be employed in the construction of residential and/or industrial structures.
In order to meet these currently desired strict requirements of flame retardancy, defined for example by oxygen indices in excess of about 40, a novel flame-retarding composition is provided for cellulosic boards wherein a synergistic effect is obtained by using a combination of an organic polyfunctional amine or amide phosphate salt with alumina hydrate. The synergistic effect between the phosphorous-containing compound and alumina hydrate is achieved only when the alumina hydrate is present in an amount corresponding to not less than about 30%, based on the total weight of the flame-retardant board. Lesser quantities of alumina hydrate at best provide only an additive flame-retarding effect with the phosphorus containing compound; unexpectedly increased flame-retarding efficiency can only be observed when the above-described minimum quantity of hydrate or more is utilized.